Photo: Creator L. RANDRIHASIPARA (CC BY 4.0) — via Wikimedia Commons
Ruby Tetra (Axelrodia riesei)
A jewel-sized blackwater tetra that glows deep crimson in a dimly lit nano tank — one of the smallest and most striking characins in the hobby.
Will it live with a Ruby Tetra?
We compare each fish against your ruby tetra on temperament, size, water parameters and swimming zone. Set your tank size and filter the results.
- Assassin Snail✅ CompatiblePeaceful · 3 cm · Easy care · 22–26 °C (72–79 °F)
- Both are peaceful, and their water overlaps around 23–26 °C — no size, zone or temperament conflicts.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Celestial Pearl Danio✅ CompatiblePeaceful · 2 cm · Medium care · 22–26 °C (72–79 °F)
- Both are peaceful, and their water overlaps around 23–26 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Celestial Pearl Danio in a shoal of 6+ or it gets stressed and nippy.
- Cherry Shrimp✅ CompatiblePeaceful · 3 cm · Easy care · 18–28 °C (64–82 °F)
- Both are peaceful, and their water overlaps around 23–28 °C — no size, zone or temperament conflicts.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Cherry Shrimp in a shoal of 10+ or it gets stressed and nippy.
- Chili Rasbora✅ CompatiblePeaceful · 2 cm · Medium care · 22–28 °C (72–82 °F)
- Compatible on the things that matter: shared water near 23–28 °C, workable temperaments, and no predator-and-prey size gap.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Chili Rasbora in a shoal of 8+ or it gets stressed and nippy.
- Crystal Red Shrimp✅ CompatiblePeaceful · 2.5 cm · Hard care · 20–24 °C (68–75 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Crystal Red Shrimp in a shoal of 10+ or it gets stressed and nippy.
- Dawn Tetra✅ CompatiblePeaceful · 2.5 cm · Easy care · 22–28 °C (72–82 °F)
- Both are peaceful, and their water overlaps around 23–28 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Dawn Tetra in a shoal of 8+ or it gets stressed and nippy.
- Dwarf Spotted Rasbora✅ CompatiblePeaceful · 2.5 cm · Medium care · 23–28 °C (73–82 °F)
- Compatible on the things that matter: shared water near 23–28 °C, workable temperaments, and no predator-and-prey size gap.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Dwarf Spotted Rasbora in a shoal of 8+ or it gets stressed and nippy.
- Ember Tetra✅ CompatiblePeaceful · 2 cm · Easy care · 24–28 °C (75–82 °F)
- Compatible on the things that matter: shared water near 24–28 °C, workable temperaments, and no predator-and-prey size gap.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Ember Tetra in a shoal of 8+ or it gets stressed and nippy.
- Emerald Dwarf Danio✅ CompatiblePeaceful · 2 cm · Medium care · 20–24 °C (68–75 °F)
- Both are peaceful, and their water overlaps around 23–24 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Emerald Dwarf Danio in a shoal of 8+ or it gets stressed and nippy.
- Exclamation Point Rasbora✅ CompatiblePeaceful · 2 cm · Medium care · 22–28 °C (72–82 °F)
- Both are peaceful, and their water overlaps around 23–28 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Exclamation Point Rasbora in a shoal of 8+ or it gets stressed and nippy.
- Glowlight Danio✅ CompatiblePeaceful · 3 cm · Easy care · 22–26 °C (72–79 °F)
- Compatible on the things that matter: shared water near 23–26 °C, workable temperaments, and no predator-and-prey size gap.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Glowlight Danio in a shoal of 8+ or it gets stressed and nippy.
- Gold Ring Danio✅ CompatiblePeaceful · 3 cm · Easy care · 18–26 °C (64–79 °F)
- Compatible on the things that matter: shared water near 23–26 °C, workable temperaments, and no predator-and-prey size gap.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Gold Ring Danio in a shoal of 8+ or it gets stressed and nippy.
- Green Neon Tetra✅ CompatiblePeaceful · 2.5 cm · Hard care · 24–28 °C (75–82 °F)
- Both are peaceful, and their water overlaps around 24–28 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Green Neon Tetra in a shoal of 8+ or it gets stressed and nippy.
- Hummingbird Tetra✅ CompatiblePeaceful · 1.8 cm · Hard care · 22–27 °C (72–81 °F)
- Both are peaceful, and their water overlaps around 23–27 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Hummingbird Tetra in a shoal of 8+ or it gets stressed and nippy.
- Lambchop Rasbora✅ CompatiblePeaceful · 3 cm · Medium care · 23–28 °C (73–82 °F)
- Compatible on the things that matter: shared water near 23–28 °C, workable temperaments, and no predator-and-prey size gap.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Lambchop Rasbora in a shoal of 6+ or it gets stressed and nippy.
- Malaysian Trumpet Snail✅ CompatiblePeaceful · 3 cm · Easy care · 21–27 °C (70–81 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Neon Blue Rasbora✅ CompatiblePeaceful · 2.5 cm · Medium care · 23–26 °C (73–79 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Neon Blue Rasbora in a shoal of 10+ or it gets stressed and nippy.
- Neon Green Rasbora✅ CompatiblePeaceful · 2 cm · Easy care · 22–26 °C (72–79 °F)
- Both are peaceful, and their water overlaps around 23–26 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Neon Green Rasbora in a shoal of 10+ or it gets stressed and nippy.
- Neon Tetra✅ CompatiblePeaceful · 3 cm · Easy care · 20–26 °C (68–79 °F)
- Both are peaceful, and their water overlaps around 23–26 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Neon Tetra in a shoal of 6+ or it gets stressed and nippy.
- Nerite Snail✅ CompatiblePeaceful · 2.5 cm · Easy care · 22–28 °C (72–82 °F)
- Compatible on the things that matter: shared water near 23–28 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Ramshorn Snail✅ CompatiblePeaceful · 2 cm · Easy care · 20–28 °C (68–82 °F)
- Compatible on the things that matter: shared water near 23–28 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Red Lip Nerite Snail✅ CompatiblePeaceful · 2 cm · Easy care · 22–28 °C (72–82 °F)
- Compatible on the things that matter: shared water near 23–28 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Strawberry Rasbora✅ CompatiblePeaceful · 2 cm · Medium care · 23–28 °C (73–82 °F)
- Both are peaceful, and their water overlaps around 23–28 °C — no size, zone or temperament conflicts.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Strawberry Rasbora in a shoal of 8+ or it gets stressed and nippy.
- Tucano Tetra✅ CompatiblePeaceful · 1.7 cm · Hard care · 23–28 °C (73–82 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Both favour the middle of the tank — offer enough cover so they aren't always in each other's space.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Tucano Tetra in a shoal of 10+ or it gets stressed and nippy.
- Black Darter Tetra⚠️ With cautionSemi-aggressive · 4 cm · Hard care · 21–28 °C (70–82 °F)
- Black Darter Tetra clearly outsizes Ruby Tetra and is semi-aggressive; risky unless the tank is big and well-planted.
- Ruby Tetra is small enough to tempt Black Darter Tetra; only risk it in a densely planted setup with hiding spots.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Black Ruby Barb⚠️ With cautionSemi-aggressive · 6 cm · Easy care · 22–26 °C (72–79 °F)
- Black Ruby Barb clearly outsizes Ruby Tetra and is semi-aggressive; risky unless the tank is big and well-planted.
- Black Ruby Barb may hunt Ruby Tetra, fry or shrimplets — safest in a heavily planted tank.
- Your 75 L tank is below the ~100 L this pairing really wants — crowding raises aggression.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Black Ruby Barb in a shoal of 6+ or it gets stressed and nippy.
- Black Skirt Tetra⚠️ With cautionSemi-aggressive · 6 cm · Easy care · 20–26 °C (68–79 °F)
- Black Skirt Tetra is semi-aggressive and may chase or nip the smaller Ruby Tetra — plant heavily and break up sight lines.
- Ruby Tetra is small enough to tempt Black Skirt Tetra; only risk it in a densely planted setup with hiding spots.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Black Skirt Tetra in a shoal of 6+ or it gets stressed and nippy.
- Blue Turbo Snail⚠️ With cautionPeaceful · 5 cm · Medium care · 25–30 °C (77–86 °F)
- Different pH ranges (5.5–7 vs 7.5–8.5); doable if you sit in the shared band, but not ideal long-term.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Desert Goby⚠️ With cautionSemi-aggressive · 6 cm · Easy care · 18–28 °C (64–82 °F)
- Desert Goby clearly outsizes Ruby Tetra and is semi-aggressive; risky unless the tank is big and well-planted.
- Watch for Desert Goby picking off any ruby tetra small enough to fit in its mouth.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Dwarf Chain Loach⚠️ With cautionPeaceful · 6 cm · Medium care · 24–29 °C (75–84 °F)
- Your 75 L tank is below the ~80 L this pairing really wants — crowding raises aggression.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Dwarf Chain Loach in a shoal of 6+ or it gets stressed and nippy.
- Endler's Livebearer⚠️ With cautionPeaceful · 3 cm · Easy care · 22–28 °C (72–82 °F)
- Water hardness preferences differ (Ruby Tetra 1–8 vs Endler's Livebearer 10–25 dGH).
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Endler's Livebearer in a shoal of 6+ or it gets stressed and nippy.
- GloFish Tetra⚠️ With cautionSemi-aggressive · 6 cm · Easy care · 21–28 °C (70–82 °F)
- GloFish Tetra clearly outsizes Ruby Tetra and is semi-aggressive; risky unless the tank is big and well-planted.
- GloFish Tetra may hunt Ruby Tetra, fry or shrimplets — safest in a heavily planted tank.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep GloFish Tetra in a shoal of 6+ or it gets stressed and nippy.
- Morse Code Corydoras⚠️ With cautionPeaceful · 5 cm · Medium care · 23–26 °C (73–79 °F)
- Your 75 L tank is below the ~80 L this pairing really wants — crowding raises aggression.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Morse Code Corydoras in a shoal of 6+ or it gets stressed and nippy.
- Odessa Barb⚠️ With cautionSemi-aggressive · 6 cm · Easy care · 20–26 °C (68–79 °F)
- Odessa Barb is semi-aggressive and may chase or nip the smaller Ruby Tetra — plant heavily and break up sight lines.
- Odessa Barb may hunt Ruby Tetra, fry or shrimplets — safest in a heavily planted tank.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Odessa Barb in a shoal of 6+ or it gets stressed and nippy.
- Pea Puffer⚠️ With cautionSemi-aggressive · 2.5 cm · Medium care · 24–28 °C (75–82 °F)
- Pea Puffer is semi-aggressive and may chase or nip the smaller Ruby Tetra — plant heavily and break up sight lines.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Rainbow Emperor Tetra⚠️ With cautionSemi-aggressive · 3.6 cm · Medium care · 22–28 °C (72–82 °F)
- Rainbow Emperor Tetra clearly outsizes Ruby Tetra and is semi-aggressive; risky unless the tank is big and well-planted.
- Rainbow Emperor Tetra may hunt Ruby Tetra, fry or shrimplets — safest in a heavily planted tank.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Rainbow Emperor Tetra in a shoal of 8+ or it gets stressed and nippy.
- Scarlet Badis⚠️ With cautionSemi-aggressive · 2 cm · Medium care · 22–27 °C (72–81 °F)
- Expect Scarlet Badis to harass Ruby Tetra at times; give dense cover and watch them at feeding.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Serpae Tetra⚠️ With cautionSemi-aggressive · 4 cm · Easy care · 23–28 °C (73–82 °F)
- Serpae Tetra is semi-aggressive and may chase or nip the smaller Ruby Tetra — plant heavily and break up sight lines.
- Ruby Tetra is small enough to tempt Serpae Tetra; only risk it in a densely planted setup with hiding spots.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Serpae Tetra in a shoal of 6+ or it gets stressed and nippy.
- Silvertip Tetra⚠️ With cautionSemi-aggressive · 5 cm · Easy care · 22–28 °C (72–82 °F)
- Expect Silvertip Tetra to harass Ruby Tetra at times; give dense cover and watch them at feeding.
- Silvertip Tetra may hunt Ruby Tetra, fry or shrimplets — safest in a heavily planted tank.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Keep Silvertip Tetra in a shoal of 6+ or it gets stressed and nippy.
- Tiger Badis⚠️ With cautionSemi-aggressive · 4 cm · Medium care · 22–24 °C (72–75 °F)
- Expect Tiger Badis to harass Ruby Tetra at times; give dense cover and watch them at feeding.
- Ruby Tetra is small enough to tempt Tiger Badis; only risk it in a densely planted setup with hiding spots.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Alligator Gar⛔ Not recommendedAggressive · 250 cm · Hard care · 24–28 °C (75–82 °F)
- Ruby Tetra is bite-sized to a 250 cm predatory alligator gar — it will be eaten.
- Alligator Gar is aggressive and may chase or nip the smaller Ruby Tetra — plant heavily and break up sight lines.
- Your 75 L tank is below the ~3785 L this pairing really wants — crowding raises aggression.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Clown Knifefish⛔ Not recommendedAggressive · 90 cm · Hard care · 24–28 °C (75–82 °F)
- Clown Knifefish (90 cm) is big enough to swallow the 2 cm Ruby Tetra whole.
- Clown Knifefish is aggressive and may chase or nip the smaller Ruby Tetra — plant heavily and break up sight lines.
- Your 75 L tank is below the ~750 L this pairing really wants — crowding raises aggression.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Fire Eel⛔ Not recommendedSemi-aggressive · 100 cm · Medium care · 24–28 °C (75–82 °F)
- Size gap is too large (100 vs 2 cm): Fire Eel will treat Ruby Tetra as food.
- Fire Eel clearly outsizes Ruby Tetra and is semi-aggressive; risky unless the tank is big and well-planted.
- Your 75 L tank is below the ~380 L this pairing really wants — crowding raises aggression.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Koi⛔ Not recommendedPeaceful · 90 cm · Medium care · 4–28 °C (39–82 °F)
- Koi (90 cm) is big enough to swallow the 2 cm Ruby Tetra whole.
- Water hardness preferences differ (Ruby Tetra 1–8 vs Koi 9–18 dGH).
- Your 75 L tank is below the ~3800 L this pairing really wants — crowding raises aggression.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Redtail Catfish⛔ Not recommendedAggressive · 120 cm · Hard care · 24–27 °C (75–81 °F)
- Ruby Tetra is bite-sized to a 120 cm predatory redtail catfish — it will be eaten.
- Expect Redtail Catfish to harass Ruby Tetra at times; give dense cover and watch them at feeding.
- Your 75 L tank is below the ~5700 L this pairing really wants — crowding raises aggression.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Spotted Gar⛔ Not recommendedAggressive · 90 cm · Hard care · 18–26 °C (64–79 °F)
- Ruby Tetra is bite-sized to a 90 cm predatory spotted gar — it will be eaten.
- Spotted Gar clearly outsizes Ruby Tetra and is aggressive; risky unless the tank is big and well-planted.
- Your 75 L tank is below the ~600 L this pairing really wants — crowding raises aggression.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Wels Catfish⛔ Not recommendedAggressive · 300 cm · Hard care · 15–25 °C (59–77 °F)
- Ruby Tetra is bite-sized to a 300 cm predatory wels catfish — it will be eaten.
- Expect Wels Catfish to harass Ruby Tetra at times; give dense cover and watch them at feeding.
- Your 75 L tank is below the ~20000 L this pairing really wants — crowding raises aggression.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
- Wolf Cichlid⛔ Not recommendedAggressive · 72 cm · Hard care · 24–30 °C (75–86 °F)
- Wolf Cichlid (72 cm) is big enough to swallow the 2 cm Ruby Tetra whole.
- Wolf Cichlid is aggressive and may chase or nip the smaller Ruby Tetra — plant heavily and break up sight lines.
- Your 75 L tank is below the ~760 L this pairing really wants — crowding raises aggression.
- Keep Ruby Tetra in a shoal of 8+ or it gets stressed and nippy.
Compatibility is computed from each species' care data — a strong starting point, not a guarantee. Individual temperament varies, so always introduce new fish slowly and watch them.
Ruby Tetra care specs
- Care level
- Medium
- Breeding
- Hard
- Max size
- 2 cm (0.8 in)
- Min tank size
- 40 L (10.6 gal)
- Temperature
- 23–28 °C (73–82 °F)
- pH
- 5.5–7
- Hardness
- 1–8 dGH
- Lifespan
- 3–5 years
- Diet
- Carnivore
- Swim level
- Middle
- Group size
- 8+ (shoaling)
- Family
- Characidae
- Origin
- Colombia — Río Meta basin (blackwater streams)
What is a Ruby Tetra?
The ruby tetra (Axelrodia riesei) is a miniature characin from Colombia’s blackwater tributaries and one of the most richly coloured nano fish available to hobbyists. Adults max out at just 2 cm (0.8 in), yet a school of eight or more in a dimly lit aquarium delivers an effect that belies their tiny size: each fish burns a deep, saturated ruby-red that shimmers against dark substrate and tannin-stained water.
Also sold as the red semolina tetra, A. riesei belongs to the family Characidae — the same broad group as neon tetras and cardinal tetras — but it occupies a very different ecological niche. Where neons tolerate a reasonably wide range of tap-water conditions, ruby tetras are committed blackwater specialists. That places them squarely in medium-difficulty territory: not impossible, but not a fish to drop into a generic community tank and hope for the best.
Where do Ruby Tetras come from?
Ruby tetras are native to Colombia, specifically to soft, slow-moving blackwater streams and forest pools of the Río Meta basin in the Orinoco drainage system. The water in these habitats is stained dark amber by decomposing leaf litter and woody debris, which releases tannins and humic acids. The result is extremely soft (1–8 dGH), mildly to moderately acidic (pH often below 6), warm (23–28 °C / 73–82 °F), and very low in dissolved minerals.
Understanding this origin is the foundation of good ruby tetra care. Every husbandry decision — substrate colour, light level, filter flow, water chemistry — should trace back to replicating those Colombian blackwater conditions as closely as practical in captivity.
What size tank do Ruby Tetras need?
The minimum tank size for ruby tetras is 40 litres (about 10 gallons), and that assumes a species-only or single-species-focus setup with very few additional inhabitants. A longer, lower tank — a standard 60 cm (24 in) or 75 cm (30 in) footprint — serves them far better than a tall, narrow column because ruby tetras are middle-water swimmers that range horizontally across the school.
A 60–75 L (15–20 gal) tank gives you more room to aquascape properly (heavy planting, driftwood, leaf litter), easier water-parameter stability, and space to keep a comfortable group of ten or more. These fish are genuinely nano-sized, so the tank does not need to be large in absolute terms — it does need to be well-designed and correctly conditioned.
Key setup elements:
- Dark substrate — fine dark sand or black aqua soil to deepen apparent colour and reduce stress from light reflection off the bottom.
- Driftwood and botanicals — spider wood, cholla wood, dried Indian almond leaves, catappa bark, and alder cones all leach tannins and buffer toward the acidic pH these fish need.
- Floating plants — frogbit, Amazon frogweed, or water sprite reduce surface light intensity and replicate the dappled canopy of their forest-stream habitat.
- Gentle filtration — a sponge filter or a hang-on-back with a spray-bar diffuser. Ruby tetras are tiny and their fins are delicate; strong current is not appropriate.
What water parameters do Ruby Tetras need?
- Temperature: 23–28 °C (73–82 °F). Stable is more important than precise; avoid swings of more than 1–2 °C between water changes.
- pH: 5.5–7.0. Ideally 6.0–6.5 for captive breeding; 6.5–7.0 is acceptable for long-term maintenance of fish not intended for breeding.
- Hardness: 1–8 dGH. Soft water is essential. In areas with hard tap water, use RO water blended back to target parameters, or collect rainwater (filtered and dechlorinated).
- Ammonia / Nitrite: 0 ppm — a fully cycled tank is mandatory before adding these fish.
- Nitrate: Keep below 20 ppm with regular water changes; lower is better.
Water change volume should be modest — around 20–25% weekly — and the replacement water must be pre-conditioned to match tank temperature and chemistry. Cold or alkaline water changes are stressful enough to bleach colour within hours.
What do Ruby Tetras eat?
Ruby tetras are carnivores in the wild, feeding on tiny invertebrates, zooplankton, and insect larvae that drift through the water column. In captivity the main feeding challenge is their size: at 2 cm (0.8 in) they simply cannot fit standard-sized foods into their mouths.
Appropriate foods include:
- Live and frozen micro-foods — baby brine shrimp (newly hatched), micro worms, daphnia, grindal worms, and vinegar eels. These should form the bulk of the diet, especially for newly imported fish.
- Dry micro-foods — high-quality micro pellets (0.5–0.8 mm) and finely crushed flake. A few brands produce colour-enhancing nano formulas that work well once fish are settled.
Feed small amounts two to three times daily rather than one larger serving. Any uneaten food should be siphoned out promptly — in a soft, low-pH tank, decomposing food can rapidly spike ammonia and strip out the buffering capacity that keeps the pH stable. Variety across the week keeps fish in good condition and supports the immune system.
Are Ruby Tetras peaceful — and what fish can live with them?
Ruby tetras are fully peaceful and pose no threat to any tankmate. The challenge runs the other way: their tiny size (2 cm / 0.8 in) and preference for subdued blackwater conditions means most common community fish are simply incompatible in practice, even if they are not aggressive by nature.
The ideal strategy is a species-only setup or a carefully curated blackwater community that matches on three axes — size (small), temperament (peaceful), and water chemistry (soft and acidic). Good companions include:
- Pencilfish (Nannostomus spp.) — elegant, nano-sized, share the same water chemistry preferences.
- Dwarf and pygmy corydoras (Corydoras pygmaeus, C. hastatus) — gentle bottom-dwellers that occupy a different tank zone.
- Pygmy rasboras (Boraras spp.) — similar size, completely peaceful, love blackwater.
- Small Apistogramma pairs — compatible in chemistry but only in tanks large enough (60 L+) that the cichlids do not monopolise the space.
- Otocinclus catfish — excellent algae control in planted tanks and entirely non-threatening.
Avoid barbs, danios, larger tetras, or any fish with a reputation for fin-nipping. Despite their tiny fins, ruby tetras do not deserve the additional stress.
For a full compatibility breakdown, see Ruby Tetra tank mates.
How do you tell male and female Ruby Tetras apart?
Sexing ruby tetras is subtle compared to many aquarium fish. Both sexes display similar deep ruby-red colouration — colour alone is not a reliable indicator. The practical guide:
- Females become noticeably fuller-bodied and rounder in the belly when gravid (carrying eggs), which is often the clearest visible distinction.
- Males are marginally slimmer through the body at all times.
- Under very good lighting and a magnifying glass, some aquarists report slight differences in the shape of the anal fin, but this is not practically useful for most keepers.
The easiest time to sex ruby tetras is after a period of good conditioning on live foods, when females will visibly swell with eggs. Outside of breeding condition, distinguishing the sexes with confidence is difficult.
How do Ruby Tetras breed?
Ruby tetras are rated hard to breed in captivity, and the difficulty is real — successful spawns outside of a dedicated breeding setup are uncommon. They are egg scatterers with no parental care and no guarding instinct; both parents will eat eggs and fry if given the opportunity.
Breeding approach for experienced keepers:
- Condition a group of six to twelve adults on heavy live food (baby brine shrimp, micro worms, daphnia) for two to four weeks in the display tank.
- Set up a dedicated spawning tank of 15–20 L (4–5 gal) with aged, RO-based soft water at pH 5.5–6.5 and temperature at the warmer end of range (26–28 °C / 79–82 °F). Fine-leafed plants or a spawning mop and a layer of marbles on the floor help protect eggs from being eaten.
- Introduce a conditioned pair in the evening. Spawning, if it occurs, typically happens in the morning over fine-leafed vegetation.
- Remove the parents immediately after spawning is observed. Eggs are tiny and clear; they hatch in approximately 24–36 hours at 27 °C.
- Raise fry on infusoria or commercially prepared fry food for the first week, then transition to freshly hatched baby brine shrimp as they grow. Maintain pristine water quality throughout, as fry are extremely sensitive.
What diseases affect Ruby Tetras?
Ruby tetras share the common disease risks of most small characins, with a few worth noting for a soft-water specialist:
- Ich (white spot disease) — tiny white cysts on the fins and body. Caused by Ichthyophthirius multifiliis; almost always introduced via new fish or equipment. Quarantine all new arrivals for four weeks before adding them to the display tank.
- Velvet (Oodinium) — a fine gold or rust-coloured dusting on the body, often mistaken for sheen. Can spread rapidly in a group of nano fish. Preventable through strict quarantine.
- Bacterial infections / fin erosion — usually secondary to poor water quality. In a correctly maintained blackwater tank with low nitrate and stable pH, incidence is low.
- Wasting / internal parasites — imported wild-caught specimens sometimes carry internal worms. Gradual weight loss despite apparent feeding is the key sign. Quarantine provides the opportunity to observe and act before introduction to the display tank.
Health note: prevention through stable soft-water chemistry, a cycled tank, regular water changes, and strict quarantine for new arrivals will prevent the great majority of disease events in a ruby tetra setup. Specific medication protocols and dosing are beyond the scope of this care profile — consult a reputable veterinary fish-health resource before treating.
How long do Ruby Tetras live?
Under good conditions, ruby tetras live 3–5 years. That is a reasonable lifespan for a 2 cm nano fish, and achieving it consistently requires the combination of factors this guide covers: correct soft-water chemistry, a calm and appropriately stocked tank, varied nutrition, and stable temperatures throughout their life.
Fish kept in unsuitable water — hard, alkaline, or brightly lit — tend to remain pale, school nervously, and rarely reach their potential lifespan. The same fish in a well-designed blackwater nano tank will hold deep colour, school confidently, and regularly prompt visitors to ask what species that striking red fish is.
Frequently asked questions
Can ruby tetras live with other fish?
Yes, but choose companions carefully. They do best with other small, peaceful blackwater species — pencilfish, dwarf corydoras, pygmy rasboras, or small apistogramma. Avoid anything large or boisterous that might outcompete them for food or nip their fins.
Why do my ruby tetras look pale?
Colour fades quickly under stress, bright light, or wrong water chemistry. They need soft, acidic water (pH 5.5–7.0), dim lighting with floating plants, and a dark substrate. Once settled in a well-conditioned blackwater setup, the ruby-red colouration returns and intensifies.
What you need to keep a ruby tetra
The baseline is a heated, filtered 40 L+ tank: a reliable heater to hold 23–28 °C (73–82 °F), a gentle filter that won't batter a ruby tetra in the current, and a tight-fitting lid. Cycle the tank fully before adding any fish.
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